1. Field of the Invention
The present invention relates generally to an exhaust gas purifying catalyst, and more particularly to an exhaust gas purifying catalyst that is able to keep a high purifying performance.
2. Description of Related Art
A lean combustion type engine such as a lean burn engine and a cylinder fuel injection type engine is run at a lean air-fuel ratio that is lean than a stoichiometrical air-fuel ratio, in a predetermined running range in order to improve a fuel consumption characteristic and an exhaust gas characteristic. While the engine is run at the lean air-fuel ratio, a three way catalyst cannot satisfactorily purify NOx (nitrogen oxide) in an exhaust gas. Therefore, it is well known that there is provided an NOx catalyst that absorbs the NOx in the exhaust gas in an oxide atmosphere, and the NOx absorbed by this catalyst is reduced to N2 (nitrogen) in an reducing atmosphere to thereby reduce the amount of NOx discharged into the air. An example of such an occlusion-type lean NOx catalyst is disclosed in Japanese Patent Provisional Publication No. 9-85093, potassium (K) as one of alkali metals is added as an NOx absorbing agent to the catalyst in order to improve an NOx absorbing performance.
If, however, the NOx catalyst to which the potassium is added is used at a high temperature for a long period of time, the catalyst may be cracked. This results in the deterioration in the durability of the NOx catalyst.
In order to discover the cause of the deterioration in the durability, the inventors of the present invention manufactured an NOx catalyst, in which potassium as one of alkali metals is added as an NOx absorbing agent to a catalyst layer held in a honeycomb cordierite carrier (a porous carrier), and conducted a bench test of an engine equipped with this NOx absorbing agent and a running test of a vehicle provided with this engine. In the bench test and the vehicle running test, the engine and the vehicle were run under the condition that the NOx catalyst was exposed to a high temperature of not less than 650xc2x0 C. for a long period of time. After the running of the engine and the vehicle, an element analysis was conducted with respect to a cut surface of the NOx catalyst by an EPMA method (an electron beam probe micro part analysis method). As a result, it was found that a compound of KMg4Al9Si9O36 of potassium, magnesium, aluminum, silicon and oxygen and a compound KAlSiO4 of potassium, aluminum, silicon and oxygen were present in a cordierite (Mg2Al4Si5O18) layer of the catalyst.
According to the above tests, if the NOx catalyst is exposed to a high temperature, the potassium added to the catalyst layer (a wash coat) permeates the cordierite carrier, and reacts with the cordierite in a high-temperature atmosphere. It can be considered that the potassium easily permeates the cordierite carrier because the potassium compound has a high water solubility and a low fusing point. When a compound with a different coefficient of thermal expansion from the cordierite is formed in the cordierite carrier, the cordierite carrier is cracked with the change in a catalyst temperature during the use of the catalyst and before and after the use of the catalyst.
As stated above, the NOx catalyst including the potassium and the like as the absorbing agent is used in the oxide atmosphere.
In the oxide atmosphere, the absorbing agent chemically reacts with nitrogen components and sulfur components in the exhaust gas to thereby form a nitrate and a sulfate of the catalyst. This deteriorates the NOx absorbing performance. The absorbing performance can be recovered by forming the reducing atmosphere around the NOx catalyst and dissolving the nitrate and the sulfate. In this case, however, the purifying performance may be deteriorated if the NOx catalyst is used at a high temperature for a long period of time.
According to the results of the tests conducted by the inventors of the present invention, one of the causes of the deterioration in the purifying performance is considered to be that the absorbing agent is gradually evaporated and splashed from the NOx catalyst at a high temperature and therefore a considerable amount of the absorbing agent is dissipated. More specifically, the inventors of the present invention manufactured the NOx catalyst in which the catalyst layer including the potassium as the absorbing agent is held in the cordierite carrier, and found a potassium content of an unused NOx catalyst by an XRF method (an X-ray fluorescence analysis method). Then, they found the potassium content of the catalyst after the use of the catalyst at a high temperature for a long period of time (e.g., at 850xc2x0 C. for 32 hours), and then found the dissipated amount of the potassium by dividing a difference in the potassium content before and after the use of the catalyst by the original potassium content. Consequently, the dissipated amount of the potassium was found to be dozens of % to 50%.
It is therefore an object of the present invention to provide an exhaust gas purifying catalyst that is able to significantly reduce the degree to which the exhaust gas purifying performance is deteriorated due to the dissipation of the absorbing agent.
It is therefore an object of the present invention to provide an exhaust gas purifying catalyst that is able to significantly reduce the degree to which the exhaust gas purifying performance is deteriorated due to the dissipation of the absorbing agent.
The above object can be accomplished by providing an exhaust gas purifying catalyst, which includes a carrier and a catalyst layer and in which at least one of alkali metals and alkali earth metals is added as an absorbing agent to the catalyst layer, the exhaust gas purifying catalyst wherein: an inhibiting agent is provided in the catalyst layer in order to inhibit the movement of the absorbing agent in the catalyst. This inhibits the movement of the absorbing agent in the catalyst, and prevents the dissipation of the absorbing agent due to the evaporation, splash, etc. of the absorbing agent from the catalyst, and the deterioration in the exhaust gas purifying performance of the catalyst.
Preferably, the inhibiting agent is dispersed and mixed in the catalyst layer, or is provided in a form of a layer in the catalyst. Only diffusing and mixing the inhibiting agent in the catalyst layer inhibits the movement of the catalyst, but the movement of the inhibiting agent can be surely prevented by providing the inhibiting agent in the form of the layer.
Preferably, the inhibiting agent includes an acid oxide including at least one acid substance selected from transition elements of IV, V and VI groups and typical elements of IV, V and VI groups; a composite oxide including the at least one acid substance; and at least one material selected from a group composed of a material that never disturbs a reactivity between a nitrogen oxide and the absorbing agent and a material that absorbs a reduced substance. In this case, the inhibiting agent may include zeolite or include an acid oxide comprised of at least one acid substance among silica, titanium and tungsten.
Preferably, the absorbing agent includes potassium, and the carrier is comprised of a porous carrier.
Moreover, the layer of the inhibiting agent preferably comprises at least one of the following: a layer with a high acidity, a layer with a large specific surface, a layer with a small crystal lattice, a layer composed of an element compound with a heavy molecular weight, and a layer with a high basicity.